The underlying physical and chemical processes that control the movement of CO2 in the subsurface of geological carbon storage (GCS) sites are complex. While building simulation models for prediction of these processes, modelers need to make various choices regarding implementation of multiphase behavior of the fluids and their equations of state, approaches for coupling of processes, modeling techniques, and selection/interpretation of site characterization and monitoring data. These model choices often lead to a wide range in predictions, even if each of the models is considering the same injection scenario at the same GCS site, and give rise to considerable uncertainties regarding the predicted behavior of CO2 storage sites.
The Sim-SEQ project was a multi-year U.S. Department of Energy initiative started in 2009 (Consolidated Sequestration Research Project (CSRP)) focused on comparing numerical models for GCS—with the objective of understanding and quantifying those uncertainties arising from conceptual model choices. It was a response to past GCS code verification and benchmarking efforts, in that it engaged in model comparison in a broader and comprehensive sense, allowing modelers the choice of interpretation of site characterization data, boundary conditions, rock and fluid properties, etc.
In Sim-SEQ, multiple modeling teams—15 different teams in all, nine of which were from outside the USA—engaged in building their own models for one specific CO2 injection field test site located at a Regional Carbon Sequestration Partnership (RCSP) CO2 injection field test site at Cranfield, in the southwestern part of Mississippi. The complex geology of the site, its location in the water leg of a CO2-EOR field with a strong water drive, and the presence of methane in the reservoir brine made this a challenging task, requiring the modelers to make a large number of choices about how to model various processes and properties of the system.
Each model team started with the same characterization data provided to them, but used its own conceptual models and simulators to come up with model predictions, which could be iteratively refined with the observation data provided to them at later stages. The assumption here was that model-building choices would be considerably more critical to model agreement than the question of which simulator to use. Model predictions were compared with one another and with the observation data, enabling the teams to better understand and quantify model uncertainties.
The project, which ended in 2014, demonstrated in an objective manner (1) that the observed system behavior at GCS sites could be predicted with confidence, and (2) that the remaining differences between models and measurements for different pilot tests, as well as between different models applied to one specific site, were well understood.
- Mukhopadhyay, S., Doughty, C., Bacon, D., Li, J., Wei, L., Yamamoto, H., Gasda, S., Hosseini, S.A., Nicot, J.P., Birkholzer, J.T. (2015): Comparison of Selected Flow Models of the S-3 Site in the Sim-SEQ Project, Transport in Porous Media, published online.
- Mukhopadhyay, S., Birkholzer, J.T., Nicot, J.P., Hosseini, S.A. (2012): A Model Comparison Initiative for a CO2 Injection Field Test: An Introduction to Sim-SEQ, Journal of Environmental Earth Sciences, Topical Issue: CLEAN – Enhanced Gas Recovery Storage and Geological CO2 Storage, 67(2), pp. 601-611.